(Position x seq) is the least index (zero-based) of the element x in
the string or list seq, if x is an element of seq. Otherwise
(position x seq) is nil. The optional keyword, :TEST, has no
effect logically, but provides the test (default eql) used for
comparing x with items of seq.

The guard for a call of position depends on the test. In all cases,
the second argument must satisfy stringp or true-listp. If the
test is eql, then either the first argument must be suitable for
eql (see eqlablep) or the second argument must satisfy
eqlable-listp. If the test is eq, then either the first argument
must be a symbol or the second argument must satisfy symbol-listp.

See equality-variants for a discussion of the relation between position and
its variants: